Inducing single-mode oscillation in FabryߝPerot InGaAsP lasers by applying external optical feedback

The authors report on inducing single-mode oscillation in InGaAsP multimode lasers with Fabry-Perot (FP) structures by applying external optical feedback (OFB). The study is based on numerical solution of a time-delay multimode rate equation model taking into account both symmetric gain suppression and asymmetric gain suppression (AGS) of modal gain. The mode dynamics and modal operation are characterised over a wide range of OFB in terms of time trajectories of the most dominant modes and the corresponding time-average output spectrum. The obtained results show that strong OFB reduces AGS relative to the linear gain in the vicinity of the dominant modes. This effect moderates the contribution of AGS to the longitudinal mode competition in such a way to induce oscillation in single mode with the laser operating in continuous wave. In this region of strong OFB, the side-mode suppression ratio increases with strengthening OFB. Increasing the external cavity length was found to decrease the OFB range of single-mode oscillation and enhances chaotic dynamics of the laser.